Heretofore, an exposure system has been widely utilized to lithographically transfer a fine circuit pattern on a wafer to fabricate integrated circuits. Since integrated circuits have been made much finer as integrated circuits have been further integrated and been provided with higher performance, such an exposure system is required to be able to provide an image of a circuit pattern on a wafer with a long focal depth and with high resolution. As a result, research and investigation have been conducted to provide an exposure light source with a shorter wavelength. As an exposure light source, an attempt will be made to employ an ArF excimer laser (having a wavelength of 193 nm) following a g-ray (having a wavelength of 436 nm), an i-ray (having a wavelength of 365 nm) and a KrF excimer laser (having a wavelength of 248 nm), which have been employed until now. In order to cope with the next generation of integrated circuits having a circuit line width of 100 nm or below, an F2 laser (having a wavelength of 157 nm) is most likely to be employed as an exposure light source. However, it has been supposed that even such an F2 laser can only cover the generation of integrated circuits having a line width of 70 nm or above.
In such a technology trend, lithographic technologies employing an EUV ray (Extreme Ultra Violet ray) as an exposure light source have drawn attention as being regarded as being applicable through the generation of integrated circuits having a line width of 50 nm and its subsequent generations. Such an EUV ray means a ray having a wavelength range in a soft X-ray region or a vacuum ultra violet region, specifically a ray having a wavelength of from about 0.2 to about 100 nm. At the present time, research and development has been conducted on the use of a wavelength of 13.5 nm as a lithographic light source. The exposure principle of the EUV lithography (hereinbelow, referred to as the “EUVL”) is the same as that of the conventional lithography in terms of transferring a mask pattern by employing a projection optical system. However, the EUVL employs a catoptric system since a dioptric system cannot be employed because of the absence of a material that allows a ray to pass therethrough in an energy region of EUV rays (see JP-A-2003-505891).
A mask employed in the EUVL is basically composed of 1) a substrate, 2) a reflective multilayer film disposed on the substrate, and 3) an absorbing material layer disposed on the reflective multilayer film. The reflective multilayer film may be configured so as to have a plurality of materials cyclically laminated on the order of nm, the materials having different refractive indexes with respect to the wavelength of an exposure ray. It has been known that typical examples of such materials include Mo and Si. Research and developments have been conducted on the use of Ta or Cr as the absorbing material layer. The substrate needs to comprise a material that has a low coefficient of thermal expansion in order to avoid distortion even under illumination of an EUV ray. Research and developments have been conducted on the use of glass having a low coefficient of thermal expansion or crystallized glass having a low coefficient of thermal expansion as the material of the substrate. The substrate may be manufactured by polishing and cleaning, with high precision, such a type of glass or crystallized glass.
In connection with a glass substrate for a magnetic recording substrate having a different application and a different function from a substrate for a reflective mask employed in such semiconductor fabrication, JP-A-10-154321 discloses that in order to prevent particles (foreign particles) from being generated, lateral surfaces (a lateral surface portion and a chamfered portion) of a glass substrate is provided with a mirror-finished surface by, e.g., mechanical polishing.